A head-mounted display (hereinafter, called as “HMD”) is a device which displays information to a user in a state that the user wears the HMD on the user's head. Generally, the HMD is desired to be compact in size and light in weight in terms of wearability, but on the other hand, is desired to be large in screen size and high in image quality in terms of display performance. Conventionally, the HMD employs a system, in which an image displayed on a compact liquid crystal panel is optically enlarged by a convex lens or a free-form surface prism, whereby an enlarged fictive image is displayed to the user (see e.g. patent literature 1). In the present specification, the aforementioned system for enlarging an image by a prism or the like is referred to as “optical enlargement system”.
Further, in a display device using a computer-generated hologram (hereinafter, called as “CGH”), a diffraction pattern obtained by using an image to be displayed as input data with use of a computer is displayed on a phase modulation type liquid crystal panel, causes laser light to irradiate the liquid crystal panel to be diffracted, whereby a wavefront of display light from a fictive image position is reproduced and the fictive image is displayed to the user (see e.g. patent literature 2). The CGH method has a feature that a three-dimensional stereoscopic image can be displayed in front of or behind the liquid crystal panel. There is also proposed a conventional example, in which a three-dimensional stereoscopic image is displayed to a user by a diffraction pattern, although this system does not employ the CGH method (see e.g. patent literature 3).
FIGS. 26A and 26B are diagrams showing an example of a diffraction pattern to be displayed on a liquid crystal panel by a CGH method, and an example of an image to be visibly recognized by the user. FIG. 26A shows an example of an original image 401. FIG. 26B shows an example of a diffraction pattern 402 generated from the original image 401. The diffraction pattern 402 is displayed on a phase modulation type liquid crystal panel or the like, the liquid crystal panel is irradiated by laser light, and the laser light is diffracted, whereby the user can visibly recognize the original image 401, based on which the diffraction pattern 402 is generated.
Generally, a generation method by a point filling method or a Fourier transform is used to compute a diffraction pattern from an original image. In the following, a computation method employing a point filling method is exemplified as a method for generating a diffraction pattern. In the point filling method, an original image (object) is defined as a group of point light sources, and a diffraction pattern is computed from the phase at which light from each of the point light sources overlap at each point on a liquid crystal panel.
FIG. 27 is a diagram showing an example of a positional relationship, in generating the diffraction pattern, between an original image 501 and a liquid crystal panel 502 on which a diffraction pattern is displayed. Each point (each pixel) on the original image 501 is defined as a point light source as described above for generating a diffraction pattern to be displayed on the liquid crystal panel 502 by a point filling method. In the case where the point “i” on the original image 501 has an amplitude “αi”, and a phase “φi”, the complex amplitude, of light from the point “i”, at the point “u” on the liquid crystal panel 502 is expressed by the formula (1).
Further, “ri” in the formula (1) denotes a distance between the point “i” and the point “u”, and is computed by the formula (2), assuming that the center of the liquid crystal panel 502 is the origin, (xi, yi, zi) denotes a coordinate of the point “i”, and (ξ, η) denotes a coordinate of the point “u”.
Further, k=2π/λ, where k in the formula (1) denotes a wavenumber, and λ denotes a wavelength of light from the point “i”. The complex amplitude, of light from the point “i”, at the point “u” is obtained by the computation based on the formula (1). Accordingly, it is possible to obtain the value of the complex amplitude at the point “u” on the liquid crystal panel 502 by performing the aforementioned computation process with respect to each of the points on the original image 501 and by summing up the computation results. The formula (3) is a computation formula representing a complex amplitude at the point “u”.
By the point filling method, a diffraction pattern is generated by performing the computation as expressed by the formula (3) with respect to each of the points on the liquid crystal panel 502. To simplify the description, in this example, a change in the phase by reference light and the like are not exemplified.
                    [                  Formula          ⁢                                          ⁢          1                ]                                                                                  u            i                    ⁡                      (                          ξ              ,              η                        )                          =                                            α              i                                      r              i                                ⁢          exp          ⁢                      {                          -                              j                ⁡                                  (                                                            k                      ⁢                                                                                          ⁢                                              r                        i                                                              +                                          ϕ                      i                                                        )                                                      }                                              (        1        )                                [                  Formula          ⁢                                          ⁢          2                ]                                                                      r          i                =                                                            (                                  ξ                  -                                      x                    i                                                  )                            2                        +                                          (                                  η                  -                                      y                    i                                                  )                            2                        +                          z              i              2                                                          (        2        )                                [                  Formula          ⁢                                          ⁢          3                ]                                                                      u          ⁡                      (                          ξ              ,              η                        )                          =                              ∑                          i              =              1                        N                    ⁢                                    u              i                        ⁡                          (                              ξ                ,                η                            )                                                          (        3        )            
However, in the case where a diffraction pattern is computed with use of a point filling method, as shown in the computation formulas (1) through (3), an increase in the pixel number of the original image 501 and an increase in the pixel number of the liquid crystal panel 502 (the pixel number of a diffraction pattern) results in an increase in the required number of times of computation, which increases the computation cost. Assuming that the pixel number of a diffraction pattern and the pixel number of an original image are both expressed by N×N (where N is a positive integer), the order of computation relating to a point filling method is the fourth power of N. Thus, as the pixel number increases, the computation amount required for computing a diffraction pattern increases.
Generally, as compared with a terminal device for use in a server or the like, the computing capacity of a mobile terminal such as an HMD is low. Accordingly, if a process requiring a large computation amount such as computing a diffraction pattern by a point filling method is performed by a mobile terminal such as an HMD, a long period of time may be necessary for generating a diffraction pattern. Further, performing a process requiring a large computation amount means considerably consuming a battery of the mobile terminal, which reduces a period of time usable for the mobile terminal.
There is proposed a method, which is an improvement of the point filling method, for computing a diffraction pattern with use of a method of applying an inverse Fourier transform to an image to be displayed to the user (see e.g. patent literature 4). However, as the pixel number of an original image or a diffraction pattern increases, the load of the computation amount of Fourier transform becomes heavy for a mobile terminal such as an HMD, which makes it difficult to generate a diffraction pattern at a high speed.
There is proposed a computation method with use of not a mobile terminal but a plurality of terminal devices having a high computing capacity to compute a diffraction pattern requiring a large amount of computation at a high speed (see e.g. non-patent literature 1). FIG. 28 is a diagram showing an example, in which a diffraction pattern is computed with use of servers having a high processability, and display is performed with use of an HMD. In the example shown in FIG. 28, servers 2602 perform computing for generating a diffraction pattern 2604 from an original image 2601. As expressed by the formulas (1) through (3), computing a diffraction pattern by a point filling method makes it easy to perform parallel processing. Accordingly, in the example shown in FIG. 28, a computing cloud 2603 is configured to allow the servers 2602 to perform parallel processing. Use of the computing cloud 2603 by the servers 2602 makes it possible to compute the diffraction pattern 2504 at a higher speed. Transmitting the computed diffraction pattern 2604 to a display terminal 2605 such as an HMD, and displaying the diffraction pattern 2604 on the display terminal 2605 makes it possible to display the diffraction pattern 2604 at a high speed, even in the case where the computing capacity of the display terminal 2605 is low.